More than 700 organic compounds have been identified in sources of drinking water in the United States (Stachka and Pontius, 1984) and elsewhere. Many water utilities, companies and government agencies must remove or destroy organic compounds from polluted groundwater supplies before those groundwater supplies can be used as drinking water. Additionally, many drinking water utilities are faced with the formation of disinfection by-products in finished water. Disinfection by-products are compounds formed in the water treatment process as a result of the disinfection process. In this process, a disinfectant such as chlorine is added to source water, where it reacts with a portion of the background organic matter (BOM) present in the source water to produce disinfection by-products. The reactive portions of the BOM are referred to as disinfection by-product precursors.
Considerable research is being directed at effective and economical treatment strategies that minimize the production of disinfection by-products. Advanced oxidation processes (AOPs) are alternative processes which destroy organic compounds and turn them into nontoxic forms, such as carbon dioxide and water. AOPs involve the generation of highly reactive radicals, such as the hydroxyl radical (OH.), which are responsible for the destruction of the organic compounds. AOPs can be classified into two major groups: AOPs involving homogeneous reactions using hydrogen peroxide (H2O2), ozone (O3), chlorine, and ultraviolet (UV) light, alone or in combination; and AOPs employing heterogeneous reactions using photoactive catalysts, such as semiconductors like titanium dioxide (TiO2) and nitrogen-doped titanium dioxide (TiO2-XNX). In the latter case—the photocatalytic oxidation processes—photoactive semiconductor catalysts are immersed in an oxygenated aqueous solution and illuminated with UV or visible radiation, so that reactive oxygen species are produced, causing the oxidation of organic compounds.
The primary oxidant responsible for the photocatalytic oxidation of organic compounds in aqueous solutions is believed to be the highly reactive hydroxyl radical (OH.), although direct reactions of adsorbed organic compounds with surface species, such as holes, have also been reported (Völz et al., 1981; Ceresa et al., Matthews, 1984; and Turchi and 011 is, 1990). When a photoactive semiconductor is illuminated with photons of the band gap energy of the semiconductor, or greater, photons excite electrons from the valence band, overcoming the energy of the band gap to the conduction band, and leaves electron vacancies, or holes, in the valence band. For example, the anatase form of TiO2 has a band-gap energy of about 3.2 eV, which is equivalent to the energy of UV light with a wavelength of 387 nm. Consequently, the anatase form of TiO2 can be activated by radiation with wavelengths less than 387 nm. The excited electrons and the resulting holes may take part in redox processes with adsorbed species, such as H2O, OH−, organic compounds and O2 at the water-solid interface. The holes may take part in oxidation half reactions with adsorbed H2O or OH− to form hydroxyl radicals. The electrons take part in the reduction half reactions with adsorbed O2 to produce the superoxide radical O2−, which may also in turn produce H2O2 and OH. (Okamoto et al., 1985).
For high photocatalytic efficiency, mesoporous TiO2 with its large surface area is highly desirable, and it was first prepared using a phosphate surfactant through a modified sol-gel process. The product was not pure TiO2 because of significant amounts of residual phosphorus, and its mesoporous structure underwent partial collapse during template removal by calcination. Another approach produced mesoporous TiO2 from amphiphilic poly(alkylene) block copolymers as structure-directing agents and organic titanium salts as precursors in a non-aqueous solution. Slight changes in reaction conditions, however, often produced very different results, rendering this method difficult to reproduce. A third method, using dodecylamine as a directing agent and titanium isopropoxide as the precursor, and emptying the pores by extractions, yielded a porous structure that was not retained after heat treatment in dry air at 300° C. Thus, it has so far been difficult to produce the highly crystalline TiO2 that is required for photocatalysis.
A second issue of current TiO2 photocatalysis technology is the requirement of ultraviolet light for activation. Because of the large energy of the band gap of TiO2 (Eg=3.2 eV in anatase), its use as a photocatalyst is limited to radiation with a wavelength of less than 380 nm. A material catalytically active when exposed to visible light of wavelengths longer than 380 nm would allow for satisfactory photocatalysis in environments where less intense light is available, for instance indoors or in a vehicle.
A further major issue of the current technology is that the powder form of the photocatalyst is difficult to handle, and too fine to be recovered from photoreactors. Thus, several films of TiO2 on various substrates and supports have been developed for photocatalytic applications. However, particle sintering and agglomeration greatly reduce the surface area of the photocatalyst.
The bonding of the TiO2 to the substrate is also a source of problems. Films of TiO2 have been assembled on substrates by direct growth and post synthetic crystal attachment. Both methods rely on chemical binders to immobilize TiO2 to the substrate surface. Unfortunately, organic binders are susceptible to decomposition under UV light. Consequently, the TiO2 films become loose from the substrate, and are easily detached.
Powders, fibers and films of TiO2 have been reported, and a number of photocatalytic TiO2 powder preparations are commercially available. However, these powders are difficult to apply to water purification, and the surface area of the powders is low, resulting in low catalytic activity and only a small number of catalytic sites.
In contrast, TiO2 fibers have a very high surface area, high wear and mechanical strength, and high thermal stability. Moreover, when used in chemical reactors, TiO2 fibers cause only a small pressure drop and can serve as a reinforcement material and as a matrix of various shapes and sizes.
TiO2 fibers may be prepared by various fabrication methods. For example, TiO2 fibers were prepared by solvothermal reaction of a fibrous K2Ti4O9 precursor, by ion-exchange reaction of K2O.4TiO7 fibers and thermal decomposition of H2Ti4O9.
Activated carbon fibers (ACF) are traditionally produced by heating an organic precursor until carbonized, and then activating the carbonized material. Activation is achieved typically by heating the carbonized material in an oxidizing environment. Alternatively, the carbon may be activated chemically. This process involves impregnating the carbon precursor with, for example, phosphoric acid, zinc chloride, or potassium hydroxide, followed by carbonization.
The above methods, however, yields brittle and frangible ACF, limiting their use to systems containing some mechanical support. This problem has been mitigated by preparing fibers where activated carbon is formed as a coating on substrate fibers.
For example, U.S. Pat. No. 5,834,114 describes glass or mineral fibers coated with activated carbon. These are prepared by coating the fiber substrate with a resin, cross-linking the resin, heating the coated fiber substrate and resin to carbonize the resin, and exposing the coated fiber substrate to an etchant to activate the coated fiber substrate.
U.S. Pat. No. 6,517,906 describes coating the substrate fibers with a mixture containing an organic polymeric material, and a chemical activating agent, for example a Lewis acid or base. This mixture carbonizes at temperatures lower than those required by earlier methods, allowing for the formation of activated carbon coatings on low melting point fibers, such as HEPA fibers.